Glycol aldehyde is a valuable intermediate useful in the synthesis of other organic compounds, and can be converted, for example, to ethylene glycol by hydrogenation. Processes for the production of glycol aldehyde by the reaction of formaldehyde with carbon monoxide and hydrogen in the presence of certain rhodium catalysts are described in the co-pending application of Alwyn Spencer, Ser. No. 256,183 filed Apr. 21, 1981, assigned to the same assignee as the present application, and corresponding European Patent EP No. 0 002 908, and also in U.S. Pat. No. 4,200,765, issued to Richard W. Goetz on Apr. 29, 1980. Also, commonly assigned copending application Ser. No. 290,622 of the present applicant and another describes the use of amines as promoters for such reactions in which tertiary organo phosphorus or arsenic groups are present as modifying ligands in rhodium catalysts. The amines improve reaction rate and permit use of high phosphine or arsine to rhodium ratios, thereby improving catalyst stability, without the excessive loss of reaction rate which would otherwise occur.